If you need to file a Self Assessment tax return for the 2025-26 tax year (6 April 2025 to 5 April 2026), missing the deadlines can be costly. HMRC charges automatic penalties for late filing that escalate quickly — even if you have no tax to pay or have paid all the tax you owe. This guide walks through every key date and explains what happens if you miss them.
Who Needs to File a Self Assessment Return?
You must register for Self Assessment and file a return if, in the 2025-26 tax year, you:
- Were self-employed and earned more than £1,000 from self-employment
- Were a company director
- Had untaxed income over £2,500 (e.g., from savings interest, rental income)
- Had total taxable income over £100,000
- Claimed the Child Benefit and either you or your partner earned over £60,000 (High Income Child Benefit Charge)
- Had foreign income on which UK tax is owed
- Were a partner in a business partnership
- Received income from a trust or settlement
If you are not sure whether you need to file, HMRC’s online tool at gov.uk can help you check. It is better to register and find out you don’t need to file than to discover you should have registered.
Key Dates for the 2025-26 Tax Year
5 October 2026 — Register for Self Assessment
If you were self-employed or had taxable income in 2025-26 and have never filed a Self Assessment return before, you must register with HMRC by 5 October 2026. Late registration doesn’t prevent you filing, but HMRC may impose a penalty. You can register online at gov.uk.
31 October 2026 — Paper Return Deadline
If you choose to submit a paper tax return, it must reach HMRC by 31 October 2026. Paper returns are less common now, but some people still prefer them. The penalty structure is the same as for online returns if you miss this deadline.
30 December 2026 — Optional Online Deadline for PAYE Taxpayers
If you are employed (PAYE) and owe less than £3,000 in additional tax, you can ask HMRC to collect the tax through your tax code in 2027-28. To do this, you must file your online return by 30 December 2026. This is optional — you can still file before 31 January 2027 and pay the tax directly instead.
31 January 2027 — Main Online Filing Deadline
This is the most important date. Your online Self Assessment return for 2025-26 must be submitted by midnight on 31 January 2027. On the same day:
- Any balancing payment for 2025-26 tax is due
- The first payment on account for 2026-27 is also due
Missing 31 January will trigger immediate penalties.
31 July 2027 — Second Payment on Account
If you make payments on account (see below), your second payment on account for 2026-27 is due by 31 July 2027.
Late Filing Penalties
HMRC imposes an automatic penalty structure for late returns. You face penalties even if you owe no tax or have already paid everything you owe.
Immediate Penalty: 1 Day Late
A £100 fixed penalty is charged automatically as soon as your return is one day late — i.e., from 1 February 2027 for the online return.
3 Months Late: Daily Penalties Begin
If your return is still outstanding after 3 months (from 1 May 2027), HMRC charges £10 per day for up to 90 days. Maximum additional penalty at this stage: £900.
6 Months Late: Percentage-Based Penalty
If the return is 6 months late (from 1 August 2027), a further penalty is charged of 5% of the tax due, or £300 — whichever is greater.
12 Months Late: A Second 5% Penalty
If the return is 12 months late (from 1 February 2028), another 5% of tax due (or £300 if greater) is charged. In cases where HMRC believes you deliberately withheld information, this can rise to 70% or 100% of the tax due.
Summary Table
| How late | Penalty |
|---|---|
| 1 day | £100 fixed penalty |
| 3 months | £10 per day (up to 90 days = max £900) |
| 6 months | 5% of tax due or £300 (whichever is greater) |
| 12 months | Further 5% of tax due or £300 (whichever is greater) |
These penalties stack, so a return filed 12 months late could result in penalties of £1,600 plus 10% of the tax due before any interest on unpaid tax.
Late Payment Penalties and Interest
If you do not pay your tax on time, HMRC also charges:
- Interest on unpaid tax from 1 February 2027 at HMRC’s current rate (see our HMRC interest rates guide)
- 5% surcharge if still unpaid after 30 days (from 2 March 2027)
- Further 5% surcharges at 6 months and 12 months
Late payment penalties and late filing penalties are separate — you can face both simultaneously.
Understanding Payment on Account
Payments on account are advance payments towards your next year’s tax bill. HMRC requires them if your last Self Assessment bill exceeded £1,000 and you paid less than 80% of your tax through PAYE or at source.
Each payment on account is half your previous year’s tax bill. They are due:
- 31 January (alongside the balancing payment for the previous year)
- 31 July
Example
Say your 2025-26 tax bill is £4,000.
- 31 January 2027: Pay £4,000 (balancing payment) + £2,000 (first payment on account for 2026-27) = £6,000
- 31 July 2027: Pay £2,000 (second payment on account for 2026-27)
If you believe your 2026-27 income will be lower, you can apply to reduce your payments on account. However, if you reduce them too much and your actual bill is higher, HMRC will charge interest on the shortfall.
Getting a Time to Pay Arrangement
If you cannot afford to pay your tax by 31 January 2027, contact HMRC before the deadline to arrange a Time to Pay (TTP) arrangement. HMRC will agree a payment schedule — usually monthly instalments — and provided you stick to it, you will not face additional late payment penalties (though interest continues to accrue).
You can set up a TTP arrangement online if you owe less than £30,000 and your return is filed. For larger amounts, call HMRC’s self-assessment helpline.
Tips to Avoid Penalties
- Register early — if you are new to Self Assessment, register by 5 October 2026
- File early — HMRC accepts returns from 6 April 2026; filing early reduces stress and gives you time to address any queries
- Keep records — HMRC can ask for evidence up to 5 years after the 31 January deadline (22 months for employed taxpayers)
- Pay promptly — even if you file on time, unpaid tax attracts interest from 1 February
- Check your UTR — you need your Unique Taxpayer Reference to file; if you have lost it, allow time to retrieve it from HMRC
Use our self-employment tax calculator to estimate your 2025-26 Self Assessment bill before the filing deadline.